Display with multilayer diffractive optical elements

ABSTRACT

The display headset for presenting an image to a user includes an electronic display and an optics block. The electronic display emits image light associated with the image toward an exit pupil corresponding to a location of an eye of the user. The optics block directs the image light from the electronic display to the exit pupil. The optics block includes a first diffractive optical element between the exit pupil and the electronic display, a second diffractive optical element between the first diffractive optical element and the exit pupil, a first protective layer on the first diffractive optical element to protect the first diffractive optical element, and a second protective layer on the second diffractive optical element to protect the second diffractive optical element. The first protective layer and the second protective layer compensate for a variation in a diffraction efficiency at different wavelengths and incident angles of the light.

BACKGROUND

The present disclosure generally relates to enhancing images fromelectronic displays, and specifically to an optical element to enhancetransmission efficiency.

A display device is an interface between an electronic device and aperson. A portable display device can be situated near eyes of a user incertain applications. For example, a display device in a form of gogglescan be placed near eyes for immersive virtual reality experience oraugmented reality experience. To present images with the display devicelocated near the eyes, a diffractive optical element (DOE) may be placedbetween human eyes and the display device to enhance the diffractionefficiency over a wide field of view. However, the conventional DOEcauses diffraction and scattering of light, hence a ghosting or a glarecan occur. Moreover, the conventional DOE can cause optical aberrationssuch as chromatic aberration, spherical aberration, coma, astigmatism,field curvature and distortion of the optical system. Accordingly, theimages presented to a user through the conventional DOE may be obscured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system environment including a virtualreality system, in accordance with an embodiment.

FIG. 2 is a diagram of a virtual reality headset, in accordance with anembodiment.

FIG. 3 is a cross section of a front rigid body of the VR headset inFIG. 2, in accordance with an embodiment.

FIG. 4 is an enlarged diagram of the optics block 318 of FIG. 3,according to an embodiment.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

SUMMARY

Embodiments relates to a display headset for presenting an image to auser wearing the display headset. The display headset can be used topresent an image of a virtual reality or an augmented reality.

In one embodiment, the display headset comprises an electronic displayconfigured to emit image light toward an exit pupil corresponding to alocation of an eye of a user; and an optics block configured to directthe image light from the electronic display to the exit pupil. Theoptics block includes a first diffractive optical element between theexit pupil and the electronic display, a second diffractive opticalelement between the first diffractive optical element and the exitpupil, a first protective layer on the first diffractive optical elementto protect the first diffractive optical element, and a secondprotective layer on the second diffractive optical element to protectthe second diffractive optical element. The first protective layer andthe second protective layer compensate for a variation in a diffractionefficiency at different wavelengths and incident angles of the imagelight.

In one or more embodiments, the first protective layer and the secondprotective layer comprise resin. The first protective layer may betransparent in a wavelength range of a visual waveband, infraredwaveband, or both.

In one or more embodiments, the first diffractive optical elementcomprises first ridges, and the second diffractive optical elementcomprises second ridges, each of the first ridges corresponding to arespective one of the second ridges. The first protective layer maycomprise third ridges substantially similar to the second ridges, andthe second protective layer may comprise fourth ridges substantiallysimilar to the first ridges. In one aspect, a refractive index of thefirst protective layer and the second protective layer is between 1.4and 1.72 or higher at a wavelength of 588 nm, wherein a refractive indexof the first diffractive optical element is between 1.4 and 1.72 orhigher at a wavelength of 588 nm, and wherein a refractive index of thesecond diffractive optical element is between 1.4 and 1.72 or higher ata wavelength of 588 nm.

In one or more embodiments, the first protective layer is formed on asurface of the first diffractive optical element facing the seconddiffractive optical element. The second protective layer may be formedon a surface of the second diffractive optical element facing the firstdiffractive optical element.

In one or more embodiment, the first protective layer and the secondprotective layer comprise a same material.

In one or more embodiments, the first diffractive optical elementcomprises a first plurality of ridges, and the second diffractiveoptical element comprises a second plurality of ridges. In one aspect, afirst surface of the first diffractive optical element faces theelectronic display; a second surface of the first diffractive opticalelement includes the first plurality of ridges toward the exit pupil; athird surface of the second diffractive optical element includes thesecond plurality of ridges toward the electronic display; and a fourthsurface of the second diffractive optical element faces the exit pupil.The first plurality of ridges may be half convex ridges and the secondplurality of ridges may be half concave ridges. The first protectivelayer may be formed on the second surface of the first diffractiveoptical element and the second protective layer may be formed on thethird surface of the second diffractive optical element.

In one or more embodiments, the optics block further includes a lenscoupled to the fourth surface of the second diffractive optical element.A surface of the lens disposed away from the fourth surface of thesecond diffractive optical element may be convex.

DETAILED DESCRIPTION

System Overview

FIG. 1 is a block diagram of a virtual reality (VR) system environment100 in which a VR console 110 operates. The system environment 100 shownby FIG. 1 comprises a VR headset 105, an imaging device 135, and a VRinput interface 140 that are each coupled to the VR console 110. WhileFIG. 1 shows an example system 100 including one VR headset 105, oneimaging device 135, and one VR input interface 140, in other embodimentsany number of these components may be included in the system 100. Forexample, there may be multiple VR headsets 105 each having an associatedVR input interface 140 and being monitored by one or more imagingdevices 135, with each VR headset 105, VR input interface 140, andimaging devices 135 communicating with the VR console 110. Inalternative configurations, different and/or additional components maybe included in the system environment 100.

The VR headset 105 is a head-mounted display that presents media to auser. Examples of media presented by the VR headset 105 include one ormore images, video, audio, or some combination thereof. In someembodiments, audio is presented via an external device (e.g., speakersand/or headphones) that receives audio information from the VR headset105, the VR console 110, or both, and presents audio data based on theaudio information. An embodiment of the VR headset 105 is furtherdescribed below in conjunction with FIG. 2. The VR headset 105 maycomprise one or more rigid bodies, which may be rigidly or non-rigidlycoupled to each other together. A rigid coupling between rigid bodiescauses the coupled rigid bodies to act as a single rigid entity. Incontrast, a non-rigid coupling between rigid bodies allows the rigidbodies to move relative to each other.

The VR headset 105 includes an electronic display 115, an optics block118, one or more locators 120, one or more position sensors 125, and aninertial measurement unit (IMU) 130. The electronic display 115 displaysimages to the user in accordance with data received from the VR console110. Example electronic display includes a liquid crystal display (LCD),an organic light emitting diode (OLED) display, an active-matrix organiclight-emitting diode display (AMOLED), a transparent OLED, some otherdisplay, or some combination thereof.

The optics block 118 magnifies received light from the electronicdisplay 115 and corrects optical errors associated with the image light.The corrected image light is presented to a user of the VR headset 105.An optical element may be an aperture, a Fresnel lens, a convex lens, aconcave lens, a filter, or any other suitable optical element thataffects the image light emitted from the electronic display 115.Moreover, the optics block 118 may include combinations of differentoptical elements. In some embodiments, one or more of the opticalelements in the optics block 118 may have one or more coatings, such asanti-reflective coatings.

Magnification of the image light by the optics block 118 allows theelectronic display 115 to be physically smaller, weigh less, and consumeless power than larger displays. Additionally, magnification mayincrease a field of view of the displayed media. For example, the fieldof view of the displayed media is such that the displayed media ispresented using almost all (e.g., 110 degrees diagonal), and in somecases all, of the user's field of view. In some embodiments, the opticsblock 118 is designed so its effective focal length is larger than thespacing to the electronic display 115, which magnifies the image lightprojected by the electronic display 115. Additionally, in someembodiments, the amount of magnification may be adjusted by adding orremoving optical elements.

The optics block 118 may be designed to correct one or more types ofoptical error. Examples of optical error include: two dimensionaloptical errors, three dimensional optical errors, or some combinationthereof. Two dimensional errors are optical aberrations that occur intwo dimensions. Example types of two dimensional errors include: barreldistortion, pincushion distortion, longitudinal chromatic aberration,transverse chromatic aberration, or any other type of two-dimensionaloptical error. Three dimensional errors are optical errors that occur inthree dimensions. Example types of three dimensional errors includespherical aberration, comatic aberration, field curvature, astigmatism,or any other type of three-dimensional optical error. In someembodiments, content provided to the electronic display 115 for displayis pre-distorted, and the optics block 118 corrects the distortion whenit receives image light from the electronic display 115 generated basedon the content.

The locators 120 are objects located in specific positions on the VRheadset 105 relative to one another and relative to a specific referencepoint on the VR headset 105. A locator 120 may be a light emitting diode(LED), a corner cube reflector, a reflective marker, a type of lightsource that contrasts with an environment in which the VR headset 105operates, or some combination thereof. In embodiments where the locators120 are active (i.e., an LED or other type of light emitting device),the locators 120 may emit light in the visible band (˜380 nm to 750 nm),in the infrared (IR) band (˜750 nm to 1 mm), in the ultraviolet band (10nm to 380 nm), some other portion of the electromagnetic spectrum, orsome combination thereof.

In some embodiments, the locators 120 are located beneath an outersurface of the VR headset 105, which is transparent to the wavelengthsof light emitted or reflected by the locators 120 or is thin enough notto substantially attenuate the wavelengths of light emitted or reflectedby the locators 120. Additionally, in some embodiments, the outersurface or other portions of the VR headset 105 are opaque in thevisible band of wavelengths of light. Thus, the locators 120 may emitlight in the IR band under an outer surface that is transparent in theIR band but opaque in the visible band.

The IMU 130 is an electronic device that generates fast calibration databased on measurement signals received from one or more of the positionsensors 125. A position sensor 125 generates one or more measurementsignals in response to motion of the VR headset 105. Examples ofposition sensors 125 include: one or more accelerometers, one or moregyroscopes, one or more magnetometers, another suitable type of sensorthat detects motion, a type of sensor used for error correction of theIMU 130, or some combination thereof. The position sensors 125 may belocated external to the IMU 130, internal to the IMU 130, or somecombination thereof.

Based on the one or more measurement signals from one or more positionsensors 125, the IMU 130 generates fast calibration data indicating anestimated position of the VR headset 105 relative to an initial positionof the VR headset 105. For example, the position sensors 125 includemultiple accelerometers to measure translational motion (forward/back,up/down, left/right) and multiple gyroscopes to measure rotationalmotion (e.g., pitch, yaw, roll). In some embodiments, the IMU 130rapidly samples the measurement signals and calculates the estimatedposition of the VR headset 105 from the sampled data. For example, theIMU 130 integrates the measurement signals received from theaccelerometers over time to estimate a velocity vector and integratesthe velocity vector over time to determine an estimated position of areference point on the VR headset 105. Alternatively, the IMU 130provides the sampled measurement signals to the VR console 110, whichdetermines the fast calibration data. The reference point is a pointthat may be used to describe the position of the VR headset 105. Whilethe reference point may generally be defined as a point in space;however, in practice the reference point is defined as a point withinthe VR headset 105 (e.g., a center of the IMU 130).

The IMU 130 receives one or more calibration parameters from the VRconsole 110. As further discussed below, the one or more calibrationparameters are used to maintain tracking of the VR headset 105. Based ona received calibration parameter, the IMU 130 may adjust one or more IMUparameters (e.g., sample rate). In some embodiments, certain calibrationparameters cause the IMU 130 to update an initial position of thereference point so it corresponds to a next calibrated position of thereference point. Updating the initial position of the reference point asthe next calibrated position of the reference point helps reduceaccumulated error associated with the determined estimated position. Theaccumulated error, also referred to as drift error, causes the estimatedposition of the reference point to “drift” away from the actual positionof the reference point over time.

The imaging device 135 generates slow calibration data in accordancewith calibration parameters received from the VR console 110. Slowcalibration data includes one or more images showing observed positionsof the locators 120 that are detectable by the imaging device 135. Theimaging device 135 may include one or more cameras, one or more videocameras, any other device capable of capturing images including one ormore of the locators 120, or some combination thereof. Additionally, theimaging device 135 may include one or more filters (e.g., used toincrease signal to noise ratio). The imaging device 135 is configured todetect light emitted or reflected from locators 120 in a field of viewof the imaging device 135. In embodiments where the locators 120 includepassive elements (e.g., a retroreflector), the imaging device 135 mayinclude a light source that illuminates some or all of the locators 120,which retro-reflect the light towards the light source in the imagingdevice 135. Slow calibration data is communicated from the imagingdevice 135 to the VR console 110, and the imaging device 135 receivesone or more calibration parameters from the VR console 110 to adjust oneor more imaging parameters (e.g., focal length, focus, frame rate, ISO,sensor temperature, shutter speed, aperture, etc.).

The VR input interface 140 is a device that allows a user to send actionrequests to the VR console 110. An action request is a request toperform a particular action. For example, an action request may be tostart or end an application or to perform a particular action within theapplication. The VR input interface 140 may include one or more inputdevices. Example input devices include: a keyboard, a mouse, a gamecontroller, or any other suitable device for receiving action requestsand communicating the received action requests to the VR console 110. Anaction request received by the VR input interface 140 is communicated tothe VR console 110, which performs an action corresponding to the actionrequest. In some embodiments, the VR input interface 140 may providehaptic feedback to the user in accordance with instructions receivedfrom the VR console 110. For example, haptic feedback is provided whenan action request is received, or the VR console 110 communicatesinstructions to the VR input interface 140 causing the VR inputinterface 140 to generate haptic feedback when the VR console 110performs an action.

The VR console 110 provides media to the VR headset 105 for presentationto the user in accordance with information received from one or more of:the imaging device 135, the VR headset 105, and the VR input interface140. In the example shown in FIG. 1, the VR console 110 includes anapplication store 145, a tracking module 150, and a virtual reality (VR)engine 155. Some embodiments of the VR console 110 have differentmodules than those described in conjunction with FIG. 1. Similarly, thefunctions further described below may be distributed among components ofthe VR console 110 in a different manner than is described here.

The application store 145 stores one or more applications for executionby the VR console 110. An application is a group of instructions, thatwhen executed by a processor, generates content for presentation to theuser. Content generated by an application may be in response to inputsreceived from the user via movement of the HR headset 105 or the VRinput interface 140. Examples of applications include: gamingapplications, conferencing applications, video playback application, orother suitable applications.

The tracking module 150 calibrates the VR system 100 using one or morecalibration parameters and may adjust one or more calibration parametersto reduce error in determination of the position of the VR headset 105.For example, the tracking module 150 adjusts the focus of the imagingdevice 135 to obtain a more accurate position for observed locators onthe VR headset 105. Moreover, calibration performed by the trackingmodule 150 also accounts for information received from the IMU 130.Additionally, if tracking of the VR headset 105 is lost (e.g., theimaging device 135 loses line of sight of at least a threshold number ofthe locators 120), the tracking module 150 re-calibrates some or all ofthe system environment 100.

The tracking module 150 tracks movements of the VR headset 105 usingslow calibration information from the imaging device 135. The trackingmodule 150 determines positions of a reference point of the VR headset105 using observed locators from the slow calibration information and amodel of the VR headset 105. The tracking module 150 also determinespositions of a reference point of the VR headset 105 using positioninformation from the fast calibration information. Additionally, in someembodiments, the tracking module 150 may use portions of the fastcalibration information, the slow calibration information, or somecombination thereof, to predict a future location of the headset 105.The tracking module 150 provides the estimated or predicted futureposition of the VR headset 105 to the VR engine 155.

The VR engine 155 executes applications within the system environment100 and receives position information, acceleration information,velocity information, predicted future positions, or some combinationthereof of the VR headset 105 from the tracking module 150. Based on thereceived information, the VR engine 155 determines content to provide tothe VR headset 105 for presentation to the user. For example, if thereceived information indicates that the user has looked to the left, theVR engine 155 generates content for the VR headset 105 that mirrors theuser's movement in a virtual environment. Additionally, the VR engine155 performs an action within an application executing on the VR console110 in response to an action request received from the VR inputinterface 140 and provides feedback to the user that the action wasperformed. The provided feedback may be visual or audible feedback viathe VR headset 105 or haptic feedback via the VR input interface 140.

FIG. 2 is a diagram of a virtual reality (VR) headset, in accordancewith an embodiment. The VR headset 200 is an embodiment of the VRheadset 105, and includes a front rigid body 205 and a band 210. Thefront rigid body 205 is configured to be situated in front of user eyes,and the band 210 is configured to be stretched and to secure the frontrigid body 205 on the user head.

In one embodiment, the front rigid body 205 is an apparatus on which animage is presented to a user. In the embodiment shown in FIG. 2, thefront rigid body 205 includes a front side 220A, a top side 220B, abottom side 220C, a right side 220D, and a left side 220E. Theelectronic display 115 is placed near the front rigid body 205. Theremaining sides (e.g., the top side 220B, bottom side 220C, right side220D and left side 220E) ensure enough distance between the electronicdisplay 115 and eyes of the user for proper presentation of the image.In one embodiment, the sides 220 of the front rigid body 205 are opaque,such that a user wearing the VR headset 200 cannot see outside of the VRheadset 200. In another embodiment, one or more of the sides 220 may betransparent.

In one embodiment, the front rigid body 205 further includes locators120, an IMU 130, and position sensors 125 for tracking a movement of theVR headset 200. The IMU 130 generates, based on motions detected by theposition sensors 125, fast calibration data which can be analyzed by theVR console 110 to determine the position of the VR headset 200, asdescribed in detail with respect to FIG. 1. The locators 120 on variousparts of the VR headset 200 are traced by the imaging device 135 atwhich slow calibration data is generated for the VR console 110 toidentify the position of the VR headset 200, as described in detail withrespect to FIG. 1.

In one embodiment, the IMU 130 is located on the front rigid body 205for generating the fast calibration data responsive to the motion of theVR headset 200 detected through the position sensors 125. In one aspect,the IMU 130 is placed on the front side 220A of the front rigid body205. Alternatively, the IMU 130 is located on any surface of the 220A ofthe front rigid body 205. In the embodiment illustrated in FIG. 2, theIMU 130 includes the position sensors 125. In other embodiments, thepositions sensors 125 may not be included in the IMU 130, and may beplaced on any side 220 of the VR headset 200.

The locators 120 are located in fixed positions on the front rigid body205 relative to one another and relative to a reference point 215. Inthe example of FIG. 2, the reference point 215 is located at the centerof the IMU 130. Locators 120, or portions of the locators 120, arelocated on a front side 220A, a top side 220B, a bottom side 220C, aright side 220D, and a left side 220E of the front rigid body 205 in theexample of FIG. 2.

FIG. 3 is a cross section of the front rigid body 205 of the embodimentof a VR headset 200 shown in FIG. 2. As shown in FIG. 3, an electronicdisplay 335 and an optics block 318 are located inside the front rigidbody 205. In one embodiment, the electronic display 335 is, e.g., theelectronic display 115 of FIG. 1, and the optics block 318 is e.g., theoptics block 118 of FIG. 1. The electronic display 335 is placed nearthe front side 220A of the front rigid body 205 facing an exit pupil350, and transmits light toward the optics block 318. The optics block318 alters image light transmitted from the electronic display 335, andprovides the altered image light to an exit pupil 350. The exit pupil350 is the location of the front rigid body 205, where a user's eye 245is positioned. Hence, light generated from the electronic display 335propagates to the exit pupil 350 through the optics block 118, forexample, via the rays 280. For purposes of illustration, FIG. 3 shows across section associated with a single eye 245, but another opticsblock, separate from the optics block 318, provides altered image lightto another eye of the user.

The optics block 318 is a component that alters light received from theelectronic display 335 and directs the altered light to the exit pupil350. In one embodiment, the optics block 318 includes diffractiveoptical elements (DOEs) 320 and 340, protective layers 325 and 345, anda lens 360. In one aspect, a first surface of the DOE 340 faces theelectronic display 335, and the protective layer 345 is coupled to asecond surface of the DOE 340 facing away from the first surface of theDOE 340. Moreover, the protective layer 325 is coupled to a firstsurface of the DOE 320 facing the DOE 340, and the lens 360 is coupledto a second surface of the DOE 320 facing away from the first surface ofthe DOE 320. The space between the protective layers 325 and 345 may bevacuum or filled with air, gas, or plastics having a certain refractiveindex to control diffractions of light for different wavelengths. Inthis structure, light transmitted from the electronic display 335propagates through the DOE 340, protective layer 345, the protectivelayer 325, the DOE 320, and the lens 360 to the exit pupil 350, in thatsequence. Together, these components of the optics block 318 direct theimage light to the exit pupil 350 for presentation to the user. In oneembodiment, the image light directed to the user may be magnified, andin some embodiments, also corrected for one or more additional opticalerrors (e.g., spherical aberration, coma, astigmatism, field curvature,distortion are third order aberrations, etc.) through the optics block318.

The DOEs 320 and 340 are formed between the exit pupil 350 and theelectronic display 335, and together form a multi-layer DOE structure.In one example, the DOEs 320 and 340 are made of plastics (e.g., PMMA,polycarbonate, E48R, etc.). When only one DOE is implemented in theoptics block 318, optical aberration such as chromatic, spherical, coma,astigmatism, field curvature and distortion may occur. By implementingtwo DOEs 320 and 340, optical aberration may be reduced.

The protective layers 325 and 345 are formed between the DOEs 320 and340. In one example, the protective layers 325 and 345 are made ofresin, or epoxy. By implementing the protective layers 325 and 345between the DOEs 320 and 340, the protective layers 325 and 345 protectrespective DOEs 320 and 340. Additionally, the protective layers 325 and345 provide an extra degree-of-freedom to compensate for the diffractionefficiency variation due to wavelengths and incident angles.Accordingly, scattering and diffraction due to the two DOEs 320 and 340can be reduced by implementing the protective layers 325 and 345.

The lens 360 is a component coupled to another surface of the DOE 320facing the exit pupil 350 for directing the light toward the exit pupil350. In one aspect, a surface of the lens 360 coupled to the secondsurface of the DOE 320 is relatively flat, whereas another surface ofthe lens 360 toward the exit pupil 350 is convex. In other embodiments,the lens 360 may have a different shape, or may be omitted.

FIG. 4 is an expanded diagram of the optics block 318 of FIG. 3,according to one embodiment. In the embodiment shown in FIG. 4, each ofthe DOEs 320 and 340 has a plurality of ridges, and each of theprotective layers 325 and 345 fills the spaces between ridges on arespective DOE. In one aspect, the protective layers 325 and 345 includematerials having a different refractive index than the refractiveindexes of the DOEs 320 and 340.

The DOEs 320 and 340 have a plurality of ridges to improve diffractionefficiency, thereby allowing the thickness and weight of the DOEs 320and 340 to be reduced. In one aspect, the DOE 340 has a first surface412 toward the electronic display 335 and a second surface 422 towardthe DOE 320 (or the exit pupil 350). In addition, the DOE 320 has afirst surface 452 toward the DOE 340 (or the electronic display 335) andthe second surface 442 toward the exit pupil 350. In one embodiment, thefirst surface 412 of the DOE 340 is a flat surface, and the secondsurface 422 of the DOE 340 includes half convex ridges 420, where a flatportion of each of the half convex ridges 420 in parallel with a centerline 400 of the optics block 318 faces the center line 400. The centerline 400 is perpendicular to the electronic display 335 and the exitpupil 250, and is located in a center of the optics block 318. Moreover,the second surface 442 of the DOE 320 is a flat surface, and the firstsurface 452 of the DOE 320 includes half concave ridges 440, where aflat portion of each of the half concave ridges 440 in parallel with thecenter line 400 of the optics block 318 faces away from the center line400. In one aspect, each of the half convex ridges 420 has asubstantially uniform height H1 and each of the half concave ridges 440has a substantially uniform height H2. A height of ridges herein refersto a maximum distance between one end of one of the ridges and anotherend of the one of the ridges along the center line 400. Preferably, theheights H1 and H2 are equal, where the shapes of the half convex ridges420 and the half concave ridges 440 correspond with each other. In otherembodiments, the shapes of the half convex ridges 420 and the halfconcave ridges 440 may not correspond with each other, or the heights H1and H2 may be different.

The protective layer 345 covers the second surface 422 of the DOE 340and the protective layer 325 covers the first surface 452 of the DOE320. In this embodiment, the height of the protective layers 345 and 325along the center line 400 are greater than the heights H1 and H2 of theridges 420 and 440, respectively. Thus, the protective layer 345includes half concave ridges 490 that fill the spaces between the halfconvex ridges 420, and the protective layer 325 includes half convexridges 480 that fill the spaces between the half concave ridges 440. Inone embodiment, shapes of the half concave ridges 490 of the protectivelayer 345 are substantially similar to shapes of respective half concaveridges 440 of the DOE 320; and similarly, shapes of the half convexridges 480 of the protective layer 325 are substantially similar toshapes of respective half convex ridges 420 of the DOE 340. In oneimplementation, a surface 482 of the protective layer 345 away from thehalf concave ridges 490, and a surface 472 of the protective layer 325away from the half concave ridges 490 are flat. In anotherimplementation, the surface 482 and 472 may be concave, convex, or acombination of both.

In one aspect, the protective layers 325 and 345 include a material withrefractive index different from the refractive index of the DOEs 320 and340. The refractive index of the protective layers 325 and 345 can bechosen such that a diffraction efficiency can be maximized.Specifically, the diffraction efficiency η_(m) can be defined asEquation 1 below:

$\begin{matrix}{\eta_{m} = {\frac{\sin\lbrack {\pi( {m - \phi} )} \rbrack}{\pi( {m - \phi} )}}^{2}} & ( {{Equation}\mspace{14mu} 1} )\end{matrix}$where ϕ is the optical path difference (OPD), in units of wavelengths,traversing the optics block 318, and m is a diffraction order. Theoptical path difference ϕ can be further expressed as equation 2 below:

$\begin{matrix}{\phi = {\frac{H_{1}\lbrack {\sqrt{{n_{m}^{2}(\lambda)} - {{n_{1}^{2}(\lambda)}{\sin^{2}(\theta)}}} - {{n_{1}(\lambda)}{\cos(\theta)}}} \rbrack}{\lambda} - \frac{H_{2}\lbrack {\sqrt{{n_{2}^{2}(\lambda)} - {{n_{1}^{2}(\lambda)}{\sin^{2}(\theta)}}} - \sqrt{{n_{m}^{2}(\lambda)} - {{n_{1}^{2}(\lambda)}{\sin^{2}(\theta)}}}} \rbrack}{\lambda}}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$where λ is the wavelength of light; θ is an incident angle (e.g., anangle between entering light and the center line 400 of FIG. 4); n₁ is arefractive index of the DOE 340; n₂ is a refractive index of the DOE320; and n₃ is a refractive index of the protective layers 325 and 345.Preferably, the optics blocks 318 is transparent in a wavelength rangebetween 400 nm to 700 nm. In one example, the optics blocks 318 can betransparent up to a wavelength of 900 nm for an eye trackingapplication. To maximize the diffraction efficiency η_(m) that isinsensitive to the wavelength λ and the incident angle θ, followingconditions can be satisfied.

$\begin{matrix}{{\phi = m},{\frac{\partial\eta_{m}}{\partial\lambda} = 0},{\frac{\partial\eta_{m}}{\partial\theta} = 0}} & ( {{Equation}\mspace{14mu} 3} )\end{matrix}$In one example, a refractive index of the protective layers 325, 345 isbetween 1.4 and 1.72 at a wavelength of 588 nm, and a refractive indexof the DOEs 320 and 340 is 1.4 and 1.72 at a wavelength of 588 nm.Preferably, the refractive indexes of the protective layers 325 and 345are the same and the refractive indexes of the DOEs 320 and 340 are thesame. Alternatively, the refractive indexes of the protective layers andDOEs can be different with each other to improve diffractive efficiency.Additional Configuration Information

The foregoing description of the embodiments has been presented for thepurpose of illustration; it is not intended to be exhaustive or to limitthe patent rights to the precise forms disclosed. Persons skilled in therelevant art can appreciate that many modifications and variations arepossible in light of the above disclosure.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the patent rights be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thepatent rights, which is set forth in the following claims.

What is claimed is:
 1. A display headset comprising: an electronicdisplay configured to emit image light in a first direction toward anexit pupil corresponding to a location of an eye of a user; and anoptics block configured to direct the image light from the electronicdisplay to the exit pupil, the optics block comprising: a firstdiffractive optical element between the exit pupil and the electronicdisplay having a first diffractive surface facing away from theelectronic display; a second diffractive optical element positionedbetween the first diffractive optical element and the exit pupil, andhaving a second diffractive surface facing the first diffractive surfaceand away from the exit pupil; a first protective layer to protect thefirst diffractive optical element, disposed on and covering the firstdiffractive surface of the first diffractive optical element facing thesecond diffractive optical element; and a second protective layer toprotect the second diffractive optical element, disposed on and coveringthe second diffractive surface of the second diffractive optical elementfacing the first diffractive optical element, the first protective layerand the second protective layer to compensate for a variation in adiffraction efficiency at different wavelengths and incident angles ofthe image light, and wherein each of the first diffractive opticalelement, the second diffractive optical element, the first protectivelayer, and the second protective layer at least partially overlap witheach other along a second direction substantially parallel to theelectronic display, and intersect with center line that is substantiallyperpendicular to a surface of the electronic display extending from thesurface of the electronic display to the exit pupil, such that the imagelight emitted from the electronic display travels through the componentsof the optics block in the order of the first diffractive opticalelement, the first protective layer, the second protective layer, andthe second diffractive optical element to reach the exit pupil.
 2. Thedisplay headset of claim 1, wherein the first protective layer and thesecond protective layer comprise resin.
 3. The display headset of claim2, wherein the first protective layer is transparent in a wavelengthrange between 400 nm to 700 nm.
 4. The display headset of claim 1,wherein the first diffractive surface of the first diffractive opticalelement comprises first ridges, and the second diffractive surface ofthe second diffractive optical element comprises second ridges, each ofthe first ridges corresponding to a respective one of the second ridges,wherein the first protective layer comprises third ridges substantiallysimilar to the second ridges of the second diffractive optical element,and wherein the second protective layer comprises fourth ridgessubstantially similar to the first ridges of the first diffractiveoptical element.
 5. The display headset of claim 4, wherein a refractiveindex of the first protective layer and the second protective layer isbetween 1.4 and 1.72, wherein a refractive index of the firstdiffractive optical element is between 1.4 and 1.72, and wherein arefractive index of the second diffractive optical element is between1.4 and 1.72.
 6. The display headset of claim 1, wherein the firstprotective layer and the second protective layer comprise a samematerial.
 7. The display headset of claim 1, wherein the firstdiffractive surface of the first diffractive optical element comprises afirst plurality of ridges.
 8. The display headset of claim 7, whereinthe second diffractive surface of the second diffractive optical elementcomprises a second plurality of ridges.
 9. The display headset of claim8, wherein the first diffractive surface of the first diffractiveoptical element that includes the first plurality of ridges faces towardthe exit pupil, and the second diffractive surface of the seconddiffractive optical element that includes the second plurality of ridgesfaces toward the electronic display.
 10. The display headset of claim 9,wherein the first plurality of ridges are half convex ridges and whereinthe second plurality of ridges are half concave ridges.
 11. The displayheadset of claim 10, further comprising: a lens coupled to a surface ofthe second diffractive optical element opposite the second diffractivesurface, a surface of the lens disposed away from the surface of thesecond diffractive optical element being convex.
 12. The display headsetof claim 8, wherein the first plurality of ridges comprises a pluralityof convex ridges, each of the convex ridges of the first plurality ofridges corresponding to a concave ridge of the second plurality ofridges.
 13. The display headset of claim of claim 12, wherein a shape ofeach of the plurality of convex ridges of the first plurality of ridgescorresponds to a shape of its corresponding concave ridge of the secondplurality of ridges.
 14. The display headset of claim of claim 12,wherein the convex ridges are half convex ridges, and the concave ridgesare half concave ridges.
 15. The display headset of claim of claim 1,wherein the first protective layer and the second protective layer arespaced apart from each other.